Abstract : The clad to coolant heat transfer is a key issue in the safety evaluation of the clad failure risk during a reactivity initiated accident RIA. It has been demonstrated that under very fast transient conditions such as those of RIA, the steady state correlations are not valid. Dedicated experiments simulating RIA conditions are required to determine the transient transfers and deeper understanding of the transient boiling phenomena is desirable. This paper presents an experimental study of the onset and development of boiling of a refrigerant flowing over a half-cylindrical wall whose heating rate is mastered. Thanks to the design of the facility, synchronized wall temperature measurement and flow visualization allow to characterize and analyse the heat transfer for the different boiling regimes. From the main results of the first tests campaigns, the impact of the wall heating rate on the boiling curve is discussed. We then draw some perspectives for further campaigns and analysis.